Meters
Feet
Litres
Gallons
SamSamWater Rainwater Harvesting ToolStep 4 of 4 (results)
Summary of results
The optimum size for a storage reservoir for this rainwater harvesting system is 6100 litres (6.1 m³).
Details on the results and calculations can be found below.
Location
Location: | |
Latitude: | 49.19851 degrees |
Longitude: | -123.99658 degrees |
Roof size: | 30 square metres |
Roof type: | metal |
Runoff coefficient: | 0.9 |
Water demand: | 100 litres per day |
Rainfall
The average rainfall at this location varies between 48.7 mm in the driest month (July) and 303.1 mm in the wettest month (December). The total annual rainfall in an average year is 1948 mm.
Water availability
A metal roof has a runoff coefficient of 0.9, which means that 90% of the rain can be harvested. Based on this runoff coefficient and a roof area of 30 square metres a volume of 1315 litres (48.7 mm x 30 m² x 0.9) of water can be collected in the driest month (July) and 8184 litres (303.1 mm x 30 m² x 0.9) in the wettest month (December).
The total yearly amount of water that can be collected from the roof is 52600 litres (53m³) in an average year.
Water demand
The water demand is 100 litres per day, which equals to about 3000 litres per month. The total water demand is 36500 litres (36.5 m³) per year.
During 7 months of the year (January, February, March, April, October, November and December) the amount of water that can be collected from the roof is larger than the water demand. This excess water can be stored to be used in the months where the water availability is smaller than the demand.
Required storage
A storage reservoir (tank) can be constructed to collect and store the water during the wet months so this water can be used during the dry months. For this location, roof size and water demand the optimum size of a storage reservoir (tank) is 6100 litres (6.1 m³).
The storage reservoir will be full in January, February, March, April, November and December and then slowly drain until it is (almost) empty at the end of September.
Dry and wet years
This calculation is based on the average monthly rainfall. The actual rainfall differs from month to month and year to year. The amount of available water and filling of the tank might therefore be different and change from year to year.
When constructing a rainwater harvesting system it is important to take this into account. Below is a description of the situation in a dry year (20% chance) and a wet year (20% chance).
Situation in a dry year: to have enough water in a dry year it is necessary to construct a larger tank for the rainwater harvesting system. A rainwater harvesting system with a reservoir of 9600 litres (9.6 m³) should provide enough water all year round in this situation. It should be decided whether or not the rainwater harvesting system should be sized to this situation or not.
Situation in a wet year: there will be more than enough water. There is no need to take additional measures.
Data source
The rainfall data used for this calculation is based on the CRU CL 2.0 dataset which is described in New, M., Lister, D., Hulme, M. and Makin, I., 2002: A high-resolution data set of surface climate over global land areas. Climate Research 21:1-25.